Thermosetting resins, such as phenolic resins, melainine resins, epoxy resins, unsaturated polyester resins, bismaleimide resins and the like, are widely applied in a variety of industry fields. However, these resins still suffer from many disadvantages, for example, the generation of volatile by-products on curing of phenolic resins or melamine resins, the poor inflammability of epoxy resins and unsaturated polyester resins and the extremely high price of bismaleimide resins. To solve the above problems, there have been made attempts to develop the thermosetting resins having the benzoxazine cyclic structure.
The resin having the benzoxazine cyclic structure has become another choice of the thermosetting resins since the thermal stability of such a resin is similar to that of phenolic resins, epoxy resins and other thermosetting resins. The thermosetting resins having the benzoxazine cyclic structure are cured by the ring-opening polymerization of the benzoxazine rings. Generally, the compounds having the benzoxazine cyclic structure is prepared by the reaction of a phenolic compound, an amine compound and an aldehyde compound. But many patents disclose the method for preparing the compounds having the benzoxazine cyclic structure, which are prepared by the reaction of aniline and phenolic compound. U.S. Pat. No. 6,005064 disclosed the thermosetting resin having the benzoxazine cyclic structure prepared by the reaction of a phenolic resin, formaldehyde and aniline; and JP-A-Hei-11-50123 also disclosed the method for preparing dihydrobenzoxazine thermosetting resin from bisphenol, aniline and formalin and using methyl ethyl acetone as a solvent. However, aniline used in these preparing method is toxic, and is a forbidden chemical material by the law. The preparation method cannot meet the requirements with the mass production in industry.
JP-A-Hei-11-50123 discloses a method for preparing dihydrobenzoxazine thermosetting resin by using methanol as a solvent and undergoing the reaction of phenol, 4,4′-diaminodiphenylmethane and paraformaldehyde. This patent only mentioned that phenol is used as a reactant, but not mentioned that a relatively pure azaoxa heterocyclic compound can be prepared by reacting phenolic compound having substituent such as alkyl, alkoxy, alkenyl groups and the like with aromatic diamine. Furthermore, methanol used in the preparing method has high polarity and has relatively large dielectric constant. When methanol is miscible with reactants in an autoclave, the gelation easily occurs and an agglomeration is formed, which results in the instability of the reaction system. On the other hand, if the temperature-control is improper in the reaction system, the compound having the benzoxazine cyclic structure formed by the condensation will further undergo polymerization due to the ring opening at the high temperature., thus resulting in low productivity and a failed reaction.
As described above, many published references disclosed the method for preparing the compound having the benzoxazine cyclic structure. However, these references all did not mention that a particular solvent is used to improve the stability of the reaction system. Therefore, the present inventors have conducted extensive studies in order to overcome the above mentioned problems. As a result, they have found that the system was endowed with relatively high stability as using the hydrocarbon solvent to conduct the reaction of phenolic compounds, aromatic diamines and aldehyde compounds. In addition to preventing the ring-reopening polymerization at high temperature due to improper temperature control, the present invention prevents the gelation or agglomeration caused by using high polar solvent or protic solvent to undergo the reaction. The azaoxa heterocyclic compound having the benzoxazine cyclic structure obtained is pure and has low water absorbency if the reaction is conducted by using a substituted phenolic compound (particularly alkyl phenolic compound) and an aromatic diamine compound as reactants instead of using high toxic aniline. The azaoxa heterocyclic compound is particularly useful in preparing laminates (CCL), copper foil adhesives, semiconductor packaging materials and phenolic resin forming materials.